When you say intermittent connectivity, are you meaning they don’t always connect or that you are getting drop outs during playback?
In the first instance a fresh pairing between the two devices is always good start point (forget them from your iPhones BT menu and re-add). You could also try a reset direct on SoundBuds by pressing the vol up/down buttons at the same time while they are turned off, this will reset any pair records on the earbuds themselves.
If the above fails to resolve it could be a possible fault with the earbuds (I’ve used the Slim with iPhone 7 with no issues) or external factors causing interference…